1. Field of the Invention
The present invention relates to dielectric ceramic compositions and dielectric ceramic compacts for radiofrequency applications, for example, dielectric ceramic compacts having high Q values in radiofrequency ranges such as microwave and millimeter-wave ranges and particularly, a dielectric ceramic composition for radiofrequency applications which can be stacked and sintered together with metal electrodes. The present invention also relates to electric components such as a filter and duplexer including the dielectric ceramic compositions or dielectric ceramic compacts.
2. Description of the Related Art
In recent years, dielectric ceramic compacts provided by sintering dielectric ceramic compositions have been widely used in radiofrequency applications, such as filters, duplexers, dielectric resonators and dielectric substrates for monolithic integrated circuits (MICs).
In order to reduce the sizes of the above products, the dielectric ceramic compositions must satisfy the following requirements:
(1) A small dielectric constant;
(2) A small dielectric loss, in other words, a high Q value; and
(3) A small dependence of the dielectric constant on temperature.
For example, Japanese Examined Patent Application Publication No. 4-59267 discloses a dielectric ceramic composition represented by the general formula (Zr,Sn)TiO4. Though this ceramic composition exhibits generally satisfactory properties: (xcex5r is at least 38 and the Q value is at least 9,000), the composition must be fired at a high temperature of at least 1,350xc2x0 C.
Use of inexpensive low-resistance metals such as Ag and Cu in internal electrode materials is required for dielectric resonators and the like in order to reduce material cost.
It is also required for simplifying the production process that the metals used in internal electrodes can be sintered together with dielectric ceramics. In order to achieve cosintering of the dielectric ceramic and the metal internal electrodes, firing must be performed at a temperature that is lower than the melting point of the metal.
The melting point of an electrode composed of Ag or Cu is generally in the range of about 960 to 1,100xc2x0 C., which is significantly lower than firing temperatures of 1,300xc2x0 C. or more applied in known firing processes for the dielectric ceramic compositions; hence, Ag and Cu cannot be used as the internal electrode materials.
The above-mentioned ceramic composition disclosed in Japanese Examined Patent Application Publication No. 4-59267 requires a high firing temperature of 1,350xc2x0 C. or more; hence, the composition cannot be sintered with low-resistance metals such as Ag and Cu.
An object of the present invention is to provide a dielectric ceramic composition for radiofrequency applications which can be fired at a low temperature and be sintered with a low-resistance metal used in internal electrodes, and a dielectric ceramic compact which exhibits superior dielectric characteristics: a high dielectric constant and a high Q value.
Another object of the present invention is to provide electronic components such as a filter comprising the dielectric ceramic composition.
According to an aspect of the present invention, a dielectric ceramic composition for radiofrequency applications comprises a crystalline primary component having a perovskite crystal structure, and an auxiliary component. The crystalline primary component is represented by the formula:
(1xe2x88x92x)MeTiaO1+2axe2x88x92xLn(Ma1/2Mb1/2)bO(3+3b)/2 
wherein Me is at least one of Ca and Sr; Ln is a rare earth element; Ma is at least one of Mg and Zn; Mb is at least one of Sn and Zr; x represents a mole fraction of Ln(Ma1/2Mb1/2)bO(3+3b/2); and a and b represent molar ratios, wherein a, b, and x are, respectively, within the following ranges: 0.95xe2x89xa6axe2x89xa61.05, 0.9xe2x89xa6bxe2x89xa61.05, and 0.3xe2x89xa6xxe2x89xa60.5. The auxiliary component comprises B and Si for decreasing the sintering temperature of the dielectric ceramic composition to about 1,000xc2x0 C. or less.
The dielectric ceramic composition satisfying the above requirements has a low sintering temperature; thus, it can be sintered with a low-resistance metal used as internal electrodes.
Furthermore, the dielectric ceramic compact obtained by sintering the dielectric ceramic composition exhibits superior radiofrequency characteristics such as the Q value and dielectric characteristics; hence, electronic components such as a filter and a duplexer using this dielectric ceramic composition exhibit superior characteristics.
Preferably, the auxiliary component comprises: about 10 to 60 percent by weight of SiO2; about 5 to 40 percent by weight of B2O3; 0 to about 30 percent by weight of Al2O3; about 20 to 70 percent by weight of EO; and 0 to about 15 percent by weight of A2O, wherein E is at least one of Zn and alkaline earth metal elements selected from Mg, Ca, Sr and Ba, and A is at least one alkali metal element selected from Li, Na and K. This dielectric ceramic composition can be more readily sintered with the low-resistance metal. Thus, the dielectric ceramic composition exhibits superior dielectric characteristics, for example, a significantly high Q value.
Preferably, the auxiliary component is a glass comprising B and Si. By adding a predetermined proportion of the glass comprising B and Si, the dielectric ceramic composition exhibits further desired characteristics.
Preferably, the content of the primary component is 100 parts by weight and the content of the auxiliary component is in the range of about 1 to 40 parts by weight. The dielectric ceramic composition for radiofrequency applications thereby exhibits desired characteristics.
Preferably, the dielectric ceramic composition further comprises an additive. The additive is more than 0 up to about 5 parts by weight of CuO or more than 0 up to about 15 parts by weight of TiO2, or both, with respect to 100 parts by weight of the primary component. The CuO additive within the above range improves sintering ability of the composition. The TiO2 additive within the above range increases the Q value.
Preferably, the rare earth element is at least one selected from the group consisting of Y, La, Pr, Nd and Sm. These rare earth elements facilitate the formation of perovskite crystals composed of the primary component, resulting in a high Q value.
Preferably, in the above formula, Me is Ca and Ma is Mg. These elements facilitate the formation of the perovskite crystal composed of the primary component, resulting in a high Q value.
According to another aspect of the present invention, an electronic component comprises a ceramic element and conductors provided in the interior of the ceramic element. The ceramic element comprises the above-described dielectric ceramic compact. This electronic component exhibits satisfactory characteristics.
In this electronic component, the ceramic element may be formed by firing a composite of a plurality of green ceramic sheets comprising the dielectric ceramic composition.
Preferably, the conductors are formed by firing respective patterns of a conductive paste applied on the respective green ceramic sheets.
The dielectric ceramic composition according to the present invention can be sintered together with a low-resistance internal electrode material at a low sintering temperature. When this dielectric ceramic composition is used in, for example, a monolithic ceramic electronic component produced through a sintering step of a composite including green ceramic sheets provided with respective internal electrode patterns composed of a conductive paste, the internal conductors can be formed by simultaneously sintering the conductive paste and the green ceramic sheets at a low sintering temperature. Thus, the resulting monolithic ceramic electronic component exhibits superior characteristics.
Preferably, the conductors comprise Ag or Cu as the major component.
Preferably, the electronic component is a filter.